1. Field of the Invention
The present invention relates to one-component sealants and adhesives hardening under the influence of moisture and based on low molecular weight, telechelic isocyanate prepolymers, whose backbone has a different chemical composition. The sealants and adhesives are preferably transparent or translucent.
Generally binary or ternary mixtures of polymers with different compositions are not compatible or have very wide miscibility gaps, i.e. only very small amounts of one component are soluble in the other component without phase separation (cf. e.g. H. G. Elias, Makromolekule, p.211, Basle, Heidelberg, N.Y., 4th edition 1980). In general, only an insignificant improvement to the compatibility is obtained through adding solvents or plasticizers to incompatible mixtures.
For the synthesis of prepolymers containing isocyanate groups and used for producing polyurethane-based adhesives and sealants, very frequently use is made of polyether polyols or polyester polyols. Standard polyether polyols are di, tri or higher functional polypropylene glycols, polyethylene glycols or copolymers thereof. Frequently used polyester polyols are prepared by the esterification of aliphatic or aromatic dicarboxylic acids with difunctional or trifunctional alcohols.
Polyurethanes are frequently used in the formulation of sealants hardening at room temperature on the one hand because they have a high hardening speed, which can easily be controlled by catalysts and on the other hand during the hardening reaction, even in the case of one-component formulations, only very small amounts of volatile constituents are released, so that the shrinkage occurring during curing is very small. This makes it possible to produce very satisfactory low modulus sealants. A further advantage is their great absorbtivity for the standard and therefore inexpensive phthalate plasticizers. Therefore very advantageous sealants can be formulated economically.
A decisive disadvantage of polyether urethanes is their very high photooxidation sensitivity, which very considerably limits the use thereof in the open in sunlight. This more particularly applies to light coloured or unpigmented sealants. In the case of outdoor exposure or the laboratory simulation thereof, the latter are very rapidly photooxidatively decomposed, so that said sealants no longer fulfil their function. It is only possible to slightly slow down such decomposition processes by adding light stabilizers, antioxidants or combinations thereof, even if they are used in unusually high doses. For this reason it is not possible to produce technically usable, unpigmented (transparent or translucent) polyurethane sealants based on polyethers.
Polyurethane polymers based on polyester polyols admittedly have a much better resistance to photooxidative decomposition than polyether urethanes, so that at a first glance they would appear to be suitable. Polyester polyols and therefore the polyurethane prepolymers produced therefrom, for a comparable molecular weight range, have a much higher viscosity than the corresponding polyether polyols and the polyurethane prepolymers produced therefrom. Two routes can be taken in spite of this to come into the viscosity range necessary for using the sealants. Firstly a very low molecular weight polyester polyol, can be used as a raw material for producing polyurethane prepolymers. However, in order that the viscosity of the polyurethane prepolymer also remains low, i.e. there is no significant molecular weight increase during the production of the prepolymer, a very high weight percentage of diisocyanate or triisocyanate is required. A sealant formulated from such a prepolymer still contains a high percentage of reactive isocyanate groups in its reactive stage. In the case of formulations without latent hardeners, i.e. formulations whose crosslinking reaction is based on the partial hydrolysis of isocyanate groups to amino groups (Hofmann reaction) and their reaction with the remaining isocyanate groups, considerable carbon dioxide quantities are split off. In non-pigmented formulations, however, no absorbents can be used for the carbon dioxide, such as e.g. calcium oxide, so that bubble and foam formation supression is impossible.
When concomitantly using blocked amines as latent hardeners, it is admittedly possible to suppress bubble formation, but due to the high reactive isocyanate group percentage, a correspondingly high latent hardener quantity is required. As a result of the large number of urea groups formed after the hardening reaction, the cured sealant becomes very hydrophilic, which is not desired in most cases.
A second possibility of obtaining a low prepolymer viscosity and therefore sealant viscosity comprises diluting with large amounts of plasticizers or solvents. As the latter evaporates from the sealant during and after the curing process, this leads to an unacceptable shrinkage of the sealant following application. It is not possible to use in the quanitity necessary for an adequate viscosity reduction of the reactive polyester urethane prepolymer the less volatile higher phthalate plasticizers, such as di-2-ethyl hexyl phthalate, dinonyl phthalate, diisodecyl phthalate, etc., because following curing plasticizer exudation occurs. The reason for this phenomenon is the high polarity of the polyester backbone of the polymer, which can be attributed to the ester-carbonyl groups. The use of polar plasticizers, such as e.g. dibutyl phthalate is not possible as a result of its high volatility and the shrinkage link therewith.
One solution of the problem would be formed by the use of mixtures of polyether polyols and polyester polyols for the purpose of producing polyurethane prepolymers with a polyether and polyester backbone, so as to combine the advantages of the two components. However, this is not possible in connection with the production of one-component sealants, because the polyester polyols in question are not compatible with the polypropylene glycols suitable for prepolymer production and also not when mixed with plasticizers. A phase separation occurs even after short storage with a very fine dispersion of the two components. A "copolymer", obtained by the joint reaction of a polypropylene glycol and a polyester polyol with a diisocyanate, is also subject to macroscopic phase separation when stored under moisture exclusion conditions.
2. Discussion of Related Art
DE-C-29 15 864 describes a moisture-hardening, one-component sealant based on a free isocyanate group-containing reaction product of a diisocyanate and a hydroxyl group-containing acrylic acid ester copolymer of low molecular weight. However, technically high grade sealants are only obtained for pigmented formulations by this process and these can also only contain a small proportion of plasticizer, based on the polymer proportion.
According to an earlier proposal (German patent application P 35 18 708.5) mixtures of polyether polyols and a number of polyester polyols compatible with low molecular weight hydroxy-functional methacrylate polymers are obtained. The preferred polyether polyols are the known di, tri or higher functional polyalcohols known to the polyurethane Expert and which are obtained by polymerizing propylene oxide or by copolymerizing propylene oxide and ethylene oxide. The commercially available polyether polyols for producing polyurethanes in the molecular weight range up to approximately 10,000 are compatible in any ratio with the described hydroxy-functional low molecular weight acrylate and methacrylate polymers. Thus, it is readily possible by reaction with diisocyanates alone to prepare homogeneous prepolymer mixtures from these two components and to use these for formulating adhesives and sealants.
However, surprisingly advantageous overall characteristics of the sealants are obtained if the polymer mixture additionally contains a suitable polyester component. Only by means of this triple composition is it possible to obtain a non-rigid, unpigmented sealant system, which also has an adequate chemical resistance and light stability to permit use for external applications.
The problem of the invention is therefore to develop a compatible binder system, which permits the production of unpigmented, preferably transparent or translucent one-component sealants. Said one-component sealant must be stable during storage, whilst excluding atmospheric humidity and after the entry of the latter must rapidly cure to give a non-rigid sealing compound. This sealant must also have an adequate stability against photooxidative decomposition, so that it can be used outside without limitations.